The invention relates to smectic liquid crystal devices.
Liquid crystal devices commonly comprise a thin layer of a liquid crystal material contained between two glass plates or walls. A thin transparent electrode is deposited on the inner face of both walls. The combination of liquid crystal layer, walls, and electrode is often termed a liquid crystal cell. When an electric field is applied between the two electrodes, liquid crystal molecules rotate in the field to an ON state. On removal of the field the molecules rotate back to an OFF state, determined by a surface treatment applied to the walls before cell assembly and by the type of liquid crystal material. The optical transmission characteristics of the ON and OFF states are different. Some devices need one or two polarisers and/or dyes to visibly distinguish between ON and OFF states.
There are broadly three different types of liquid crystal material, nematic, cholesteric, and smectic, each distinguished by a different molecular ordering.
Such materials only show a liquid crystal phase over a limited temperature range between the solid and isotropic liquid phases. Within the liquid crystal phase temperature range a material may exhibit one or more of the nematic, cholesteric or smectic phase types. Normally a material is chosen such that it forms only one type of liquid crystal phase over its working temperature range.
The present invention concerns devices using smectic liquid crystal materials.
Displays have been made with the electrodes formed These collectively form an x, y matrix of separately addressable elements on a large display. One way of addressing such a display is by multiplexing i.e. addressing each row in sequence until the whole display has been addressed. This is repeated all the time that a display is needed. Another type of display uses the ON and OFF states to form an electrically switchable optical shutter. Yet another type of display is used as an optical storage device. Nematic cholesteric and smectic liquid crystal material have been used for such devices. A problem with many displays is the time taken to switch between the two states i.e. the response times. For many displays a fast response time is needed. A nematic material, arranged in a 90xc2x0 twisted structure typically has a response time of 100 milliseconds.
Devices comprising smectic materials have not been used as widely as devices with nematic or cholesteric materials. Available display devices based on smectic materials did not have the necessary characteristics. Recently however ferro electric smectic devices with fast switching and bistability characteristics have become of interest. N. A. Clark and S. T. Lagerwall, App. Phys. Letters 35 (11) 1090 pp 899-901. Chiral liquid crystal materials in a tilted smectic phase, e.g. SC*, SI*, SF*, SJ*, SG*, SH* are known to exhibit ferroelectric properties. This has been described by R. B. Meyer, L. Liebert, L. Strzelecki and P. Keller, J. de Physique (Lett), 36, L-69 (1975).
According to this invention a method of making a liquid crystal display device comprises the steps;
providing two cell walls spaced apart by a spacer to contain a layer of a liquid crystal material, the wall inner surfaces having formed thereon electrode structures and at least one wall surface treated to provide a liquid crystal alignment,
providing a tilted chiral smectic liquid crystal material having a cholesteric phase at an elevated temperature above ambient between the chiral smectic and isotropic phases with a cholesteric pitch p greater than half the layer thickness d at a temperature at least 0.1xc2x0 C. above a cholesteric to smectic transition temperature and a significant spontaneous polarization Ps in the chiral smectic phase,
heating the mixture to the cholesteric phase,
introducing and sealing the mixture into the space between the walls,
cooling the material to the tilted chiral smectic phase.
The heating step may be before or after introduction of the material to the space.
The liquid crystal material may be a single component or a mixture of components. The material may combine one or more chiral components with laevo (left hand) cholesteric twist sense with one or more chiral components with dextro (right hand) cholesteric twist sense. With such a mixture none of the laevo components can be the racemates of the dextro components. This chiral mixture may be a chiral smectic itself or may be used as an additive to a non-chiral and/or racemate tilted smectic liquid crystal host material. Alternatively the chiral component or components may have the same cholesteric twist sense providing the cholesteric pitch and Ps have the above values.
The smectic phase has at least one of the following chiral tilted smectic phases C*, I*, F*, J*, G*, K*, H* depending on temperature and may include a smectic A phase between the cholesteric to chiral smectic phase transition.
The layer thickness may be up to 15 xcexcm or more, but is typically 1-12 xcexcm, e.g. 2 xcexcm and 6 xcexcm.
The p/d ratio is large enough to prevent the material, in its cholesteric phase close to the cholesteric to smectic phase transition, adopting more than a 3 xcfx80 twist, ideally the pitch p is greater than 4d, so that less than xcfx80 twist can occur in the cholesteric phase near the transition. Ideally this value of p is large over the whole cholesteric phase, preferably is large for 5xc2x0 C. above the smectic/cholesteric transition.
The pitch in the smectic phase is greater than 0.1 xcexcm and preferably much greater, e.g. greater than layer thickness.
The value of Ps is at least 0.1 and preferably 1 or more nC/cm2. All the chiral components may have substantial values of Ps and the same sense of Ps. Alternatively one or more components may have Ps values of opposite senses providing the net Ps is substantial.
For some mixtures the cooling is at a rate of between 0.05xc2x0 and 2xc2x0 C./minute within +/xe2x88x925xc2x0 C. of the cholesteric to smectic phase transition. The rate of cooling depends upon the amount of compensation in the cholesteric phase. For well compensated material the cell may be cooled as quickly as is convenient, e.g. 20xc2x0 C./minute or more.
According to this invention a liquid crystal device comprises a layer of a tilted chiral smectic liquid crystal material contained between two cell walls each carrying electrode structures and at least one wall surface treated to give alignment to the liquid crystal molecules, a first linear polarizer and either a second polarizer or an amount of dichroic dye in the liquid crystal material,
a liquid crystal material having a tilted chiral smectic phase at normal device operating temperatures and a cholesteric phase at a higher, above ambient, temperature, with a cholesteric pitch greater than half the layer thickness in the cholesteric phase for at least 0.1xc2x0 C. above the cholesteric to smectic transition temperature, and a substantial spontaneous polarization.
The device cell may be arranged between two polarizer arranged with their optical axes parallel or at a non zero angle to one another. Alternatively the liquid crystal mixture may include an amount of a dichroic dye. In this case only one polariser is needed.
The device may further include means for applying two d.c. voltages of opposite polarity to the electrode structures whereby the molecules of the mixture are forced to lie in one of two different positions depending on the polarity of applied voltage and switch between these two positions as the polarity is changed.
The cell wall surface treatment may provide similar alignment, in the same or different directions, or different types of alignment, e.g. homogeneous and homoetropic, on the two walls. Alternatively one wall may be left cleaned but with no surface alignment.
According to this invention a liquid material mixture for use in a tilted chiral smectic liquid crystal device comprises a material having a tilted chiral smectic phase at normal device operating temperatures and a cholesteric phase at an elevated temperature, the mixture having a cholesteric pitch greater than 4 xcexcm in the cholesteric phase at least 0.1xc2x0 C. above a cholesteric to smectic transition temperature, and a substantial spontaneous polarization coefficient Ps, in the smectic phase.
The device operating temperature typically falls within the range of 0xc2x0 to 40xc2x0 C. although for some devices mounted on equipment the upper operating temperature may be about 100xc2x0 C. or even higher.
Use of chiral components in nematic liquid crystal material is well known. The addition of a chiral component imparts a twist direction to the nematic material director and results in a cholesteric material. The direction of this twist can either be clockwise or anticlockwise, i.e. right or left rotation. Adding two different chiral materials of opposite twist can result in zero twist depending upon the composition and temperature of the mixture. Some compounds can have molecules with both right and left rotary power, these are optical isomers when the optical isomers are present in equal amounts a racemic mixture is formed with is indistinguisable from ordinary non-chiral liquid crystalline materials.